Method for forming a thick-film resistor and thick-film resistor formed thereby

ABSTRACT

A process for forming a thick-film resistor whose dimensions can be accurately obtained, thereby yielding a precise resistance value. The method includes providing on a substrate a photoimageable layer that preferably forms a permanent dielectric layer of a multilayer structure. An opening is photodefined in the surface of the photoimageable layer, and then overfilled with an electrically-resistive material to form a resistive mass having an excess portion that lies on the surface of the photoimageable layer surrounding the opening. Following curing which causes the surface of the resistive material to become recessed below the surface of the photoimageable layer, the excess portion of the resistive mass is removed, preferably by abrading or a similar operation, such that the lateral dimensions of the resistive mass are determined by the lateral dimensions of the opening in the photoimageable layer. Thereafter, subsequent processing is performed to include the photoimageable layer as a permanent photoimageable layer of a circuit board, with the resistive mass and appropriate terminations forming a resistor in the permanent photoimageable layer.

The present application is Divisional of prior U.S. application Ser. No.09/093,007 filed on Jun. 5, 1998 now U.S. Pat. No. 6,171,921, which ishereby incorporated by reference, and priority thereto for commonsubject matter is hereby claimed.

This invention was made with Government support under Agreement No.F33615-96-2-1838 awarded by DARPA. The Government has certain rights inthe invention.

BACKGROUND OF THE INVENTION

The present invention generally relates to printed wire board circuitsand their fabrication. More particularly, this invention relates to amethod for forming a thick-film resistor to have precise dimensionsdetermined by photolithography techniques, thereby avoiding thevariability associated with conventional screen printed resistors.

Thick-film resistors are employed in hybrid electronic circuits toprovide a wide range of resistor values. Such resistors are formed byprinting, such as screen printing, a thick-film resistive paste or inkon a substrate, which may be a printed wiring board (PWB), flexiblecircuit, or a ceramic or silicon substrate. Thick-film inks used withceramic printed wire boards are typically composed of a glass fritcomposition, an electrically-conductive material, various additives usedto favorably affect the final electrical properties of the resistor, andan organic vehicle or polymer matrix material. Thick-film inks used inorganic printed wire board construction are typically composed of anelectrically-conductive material, various additives used to favorablyaffect the final electrical properties of the resistor, an organicbinder and an organic vehicle. After printing, the thick-film ink istypically heated to dry the ink and convert it into a suitable film thatadheres to the substrate. If a polymer thick-film ink is used, theheating step serves to remove the organic vehicle and to cure thepolymer matrix material. Other thick-film inks must be sintered, orfired, during which the ink is heated to burn off the organic vehicleand fuse the remaining solid material.

The electrical resistance of a thick-film resistor is dependent on theprecision with which the resistor is produced, the stability of theresistor material, and the stability of the resistor terminations.Control of the x, y and z dimensions (the width, electrical length andthickness, respectively, of the resistor) is particularly challenging inview of the techniques employed to print thick-film inks and thedimensional instability that may occur during subsequent processing. Forrectangular screen-printed resistors, the x and z dimensions aredetermined by the screening process, and the y dimension is determinedby the termination pattern. Conventional screen printing techniquesgenerally employ a template with apertures bearing the positive image ofthe resistor to be created. The template, referred to as a screeningmask, is placed above and in close proximity to the surface of thesubstrate on which the resistor is to be formed. The mask is then loadedwith the resistive ink, and a squeegee blade is drawn across the surfaceof the mask to press the ink through the apertures and onto the surfaceof the substrate.

Compared to many other deposition processes, screen printing is arelatively crude process. As a result, screen printed thick-filmresistors are typically limited to dimensions of larger than about onemillimeter, with dimensional tolerances generally being larger thanabout ±10% at this lower limit. Consequently, screen printed thick-filmresistors having adequate tolerances in the x and y dimensions are oftenlarger than chip resistors. The thickness of a thick-film resistor cangenerally be controlled to tolerances of about 20% to 30% by screenprinting, due in large part to variability in the x, y and z dimensions.While the z dimension (thickness) of a screen-printed thick-filmresistor can be reasonably well controlled through precision in thescreening operation, the control of x and y dimensions is fundamentallylimited by the relatively coarse mesh of the screen and by ink flowafter deposition. As a result, resistance tolerances of less than ±20%cannot be achieved with screen printed thick-film resistors withoutlaser trimming, an operation that is usually cost prohibitive forcomplex circuits.

From the above, it can be seen that present practices involving thefabrication of thick-film resistors can necessitate a compromise betweenthe precision of the resistance value and the size of the resistor. Inother words, while smaller resistors are often preferred to yield a morecompact circuit, an undesirable consequence is that resistance valuesare less predictable due to the dimensional variability of theresistors. Accordingly, a need exists for a method for producing athick-film resistor in which resistance values and tolerances can bemore accurately controlled than prior art screen printing techniquespermit.

SUMMARY OF THE INVENTION

According to the present invention, a process is provided for forming aresistor whose x (width), y (electrical length) and z (thickness)dimensions can be accurately obtained, thereby yielding a resistor whoseresistance value can be precisely obtained even for resistors havingdimensions on the order of about 100 micrometers. The resistor can beformed with conventional thick-film resistor inks used to form screenprinted thick-film resistors, although the precision of the finaldimensions of resistors formed in accordance with this invention isbetter than that possible for prior-art screen printed thick-filmresistors.

The method of this invention preferably entails the use of one or morephotoimageable resins as masks that may subsequently form permanentdielectric layers of a multilayer structure. The method includesproviding a dielectric layer on a substrate, and then defining anopening in the surface of the dielectric layer. In the preferredembodiment where the dielectric layer is formed with a photoimageableresin, the opening is photodefined to precisely achieve the dimensionsfor the intended thick-film resistor. The opening is then filled with anelectrically-resistive material, preferably using a screen printingtechnique with an oversized aperture such that theelectrically-resistive material forms a resistive mass having an excessportion that lies on the surface of the dielectric layer surrounding theopening. The excess portion of the resistive mass is then selectivelyremoved such that the resistive mass has lateral dimensions defined bythe opening in the dielectric layer. Thereafter, subsequent processingis performed such that the dielectric layer forms a permanent layer of acircuit board, with the resistive mass and appropriate terminationsforming a resistor in the permanent dielectric layer.

In the preferred embodiment, the portion of the resistive mass lying onthe surface of the dielectric layer is removed by abrading, such asgrinding or polishing. For this purpose, the resistive mass is heated tocure the electrically-resistive material prior to the abradingoperation, such that shrinkage of the resistive mass occurs and thesurface of the resistive mass becomes recessed below the surface of thedielectric layer. During the abrading operation, the excess portion ofthe resistive mass remaining on the surface of the dielectric layer isremoved without substantially altering the recessed surface of theresistive mass and with minimal removal of the dielectric layer.

As is conventional, the terminations of the thick-film resistor of thisinvention determine the electrical length of the resistor. Theterminations may be in the form of electrical contacts lying eitherabove or below the resistor. To form terminations that overlie theresistor, the method of this invention further includes the step offorming a second dielectric layer on the dielectric layer and over theresistive mass after the excess portion of the resistive mass has beenremoved. Openings are formed in this second dielectric layer to exposeat least two regions of the resistive mass, and anelectrically-conductive material is then deposited in the openings toform electrical contacts that serve as terminations for the resistor.Alternatively, terminations that lie below the resistor may be formed inor on the substrate underlying the dielectric layer in which theresistor is formed.

From the above, those skilled in the art will appreciate that the methoddescribed above is conducive to inline processing, thus enabling highthroughput and short cycle times. More significantly, those skilled inthe art will appreciate that the method of this invention more preciselyproduces thick-film resistors than possible with prior art screenprinting techniques, because the opening formed in the dielectric layerand the openings for the terminations determine the width and electricallength, respectively, of the thick-film resistor. Meanwhile, the screenprinting operation is limited to determining the thickness of thethick-film resistor. According to the invention, dielectric layersformed of photoimageable thick-film polymers are particularly suitablefor confining a mass of resistor ink, such as polymer thick-film inksknown in the art. The precision with which the openings for the resistorand terminations can be photolithographically defined yields athick-film resistor whose dimensions can be controlled far better thanis possible by conventional screen printing techniques. As a result,thick-film resistors having dimensions on the order of about 100micrometers with dimensional tolerances of less than about ±5% arepossible with this invention. Notably, the method of this inventionentails depositing the resistive ink in and around the opening in thedielectric layer, a step that can be readily carried out by screenprinting because the screen printing operation does not itself determinethe lateral dimensions of the resulting thick-film resistor. Incontrast, prior art screen printing methods have relied solely on thedimensions of an aperture in a screening mask to determine and controlall dimensions of a thick-film resistor, which has limited prior artscreen printed thick-film resistors to dimensions larger than about onemillimeter with dimensional tolerances generally larger than about ±10%at the lower size limit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 through FIG. 3 represent processing steps for forming thick-filmresistors in accordance with a preferred embodiment of this invention;and

FIG. 4 and FIG. 5 are cross-sectional views of thick-film resistors withalternative terminal configurations in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Processing steps for producing a thick-film resistor in accordance withthis invention are generally represented in FIGS. 1 through 3, withterminal patterns suitable for the thick-film resistor being shown inFIG. 4 and FIG. 5. The processes described and represented in the FIG. 1through FIG. 3 achieve the advantageous features of this invention inreference to forming thick-film resistors having precise dimensions,though it will be apparent to those skilled in the art that theinvention is applicable to other thick-film circuit components,including capacitors. Furthermore, while particular resistorconfigurations are shown in the FIG. 4 and FIG. 5, those skilled in theart will appreciate that numerous variations and modifications arepossible, and such variations and modifications are within the scope ofthis invention.

Referring to FIG. 1, a substrate 10 is shown on which a dielectric layer12 has been formed. The substrate 10 can be any suitable material,including a printed wiring board, a flexible circuit, a ceramic orsilicon substrate, or another dielectric layer of a multilayer circuit,though other suitable substrates and materials could also be used. Asshown, a resistive ink 14 has been deposited in an opening 16 preciselyformed in the dielectric layer 12, so that the width and length of athick-film resistive mass 18 (FIG. 2 through FIG. 5) formed from the ink14 will have the precise dimensions of the opening 16. For this purpose,the dielectric layer 12 is preferably formed of a photoimagablethick-film polymer, such that known photoimaging and developmenttechniques can be employed to pattern the opening 16 in the dielectriclayer 12. Suitable thick-film polymer compositions typically include aresin, photosensitive agents and hardeners. The resin component can beany suitable liquid resin or solid resin, so as to enable the resinmixture to be readily deposited onto the surface of the substrate 10 inliquid form, a spreadable form, or as a laminate to form the dielectriclayer 12. Resins that could be used include thermoplastic resins,thermosetting resins, elastomers and mixtures thereof, which whenincorporated with a photosensitive material yield a photoimageablecomposition. Desirable properties for the thin-film polymer includephysical properties that remain stable throughout deposition andphotoimaging of the dielectric layer 12. If, as in the preferredembodiment, the dielectric layer 12 is to serve as a permanentdielectric layer of the circuit structure on the substrate 10, thedielectric properties of the thin-film polymer also preferably remainstable throughout the deposition and photoimaging processes. For theabove reasons, epoxies are particularly suitable as the resin for thedielectric layer 12, with preferred epoxy-base compositions being LMB7081 and LMB 7082 commercially available from Ciba-Geigy.

In a preferred embodiment, the dielectric layer 12 is applied as one ormore films to yield a precise thickness of about fifteen to about fortymicrometers, although greater and lesser thicknesses are possible. Dueto the presence of photosensitive agents, the opening 16 in thedielectric layer 12 can be precisely photochemically defined through amask (not shown) and developed in the dielectric layer 12. As shown inFIG. 1, the opening 16 extends through the dielectric layer 12 to exposea portion of the substrate 10. The resistive ink 14 is then deposited inthe opening 16 by screen printing, stenciling or any other techniquecapable of depositing a controlled amount of ink 14 in and around theopening 16. In and around the opening signifies on the edge of theopening 16 and the surface of the dielectric layer 12 immediatelysurrounding the opening 16, as shown in FIG. 1. Suitable inkcompositions are polymer thick-film inks containing conductiveparticulate fillers dispersed in a polymeric matrix. A preferred inkcomposition known in the art contains carbon particles as the fillerdispersed in a heat-curable polymer, preferably a phenolic resin. Inalternate embodiments, the conductive metallic particulate filler maycomprise powder.

The resistor is preferably made from a resistive ink or a resistivematerial with a sheet resistivity within a range extending fromapproximately 1 kiloohms per square to approximately 100 kiloohms persquare. The applied resistive ink typically has a precured thicknessranging from approximately fifteen to approximately forty micrometers,although greater or lesser thicknesses are possible. The cured resistivematerial may have a resistivity which ranges from approximately 5ohm-centimeters to approximately 100 ohm-centimeters.

A preferred technique for depositing the resistive ink 14 is to employ ascreening mask (not shown) with an aperture whose lateral dimensions arelarger than those of the opening 16, with the aperture being centeredover the opening 16 such that the ink 14 is deposited on the entire edgeof the dielectric layer 12 surrounding the opening 16, as well as in theopening 16. Preferably, as shown in FIG. 1, the resistive ink 14 isdeposited in an amount sufficient to fill or nearly fill the opening 16without overfilling the opening 16, other than the ink 14 depositedaround the opening 16 on the dielectric layer 12. In so doing, heatingthe ink 14 to cure the polymer matrix of the resistive ink 14 yields acured resistive mass 18 whose interior upper surface within the opening16 is recessed below the surface of the dielectric layer 12, as seen inFIG. 2. In addition, as a result of ink 14 having been deposited on thedielectric layer 12 surrounding the opening 16, the resistive mass 18has an excess portion 19 on the surface of the dielectric layer 12surrounding the opening 16, as shown in FIG. 2. This excess portion 19is subsequently removed to yield the configuration for the resistivemass 12 shown in FIG. 3.

Importantly, removal of the excess portion 19 of the resistive mass 18following curing enables the lateral dimensions of the resistive mass 18to be precisely determined by the length and width of the opening 16,whose precise dimensions are provided by photoimaging. The portion 19 ofthe cured resistive mass 18 that lies on the surface of the dielectriclayer 12 can be readily removed by milling or another planarizingtechnique, such that the peripheral extent of the resistive mass 18terminates at the surface of the dielectric layer 12. In contrast, theinterior upper surface of the resistive mass 18 is unaltered duringplanarizing because the surface is recessed below the surface of thedielectric layer 12 due to shrinkage that occurred during curing, as isevident from FIG. 2 and FIG. 3. In a preferred embodiment, the excessportion 19 of the cured resistive mass 18 is removed by an abradingoperation, such as grinding or polishing, which can serve topreferentially and rapidly remove the excess portion 19 lying on thesurface of the dielectric layer 12. According to this invention, theabrading operation can be performed with conventional deburring,polishing or grinding equipment.

Following the abrading operation, terminals 20 can be formed to overliethe resistive mass 18, as shown in FIG. 4. A preferred process is todeposit a second photoimageable layer 22, and then pattern thephotoimageable layer 22 to photolithographically form openings 24 asshown. The photoimageable layer 22 can be formed from a variety ofmaterials, including those similar or compatible to that of the firstdielectric layer 12. The photoimageable layer 22 may but is not requiredto serve as a permanent dielectric layer of the circuit structure. Theterminals 20 can then be selectively formed through the openings 24 onthe resistive mass 18, so that the terminals 20 lie above the plane ofthe resistive mass 18. Preferably, the terminals 20 include a nickel,gold, silver, palladium or platinum contact 26 between the resistivemass 18 and a copper interconnect 28. A suitable contact 26 iselectroless nickel 26 having a thickness of about 0.1 to about 5micrometers.

As shown in FIG. 5, an alternative termination technique within thescope of this invention is to form the terminals 20 on the surface ofthe substrate 10 prior to applying the dielectric layer 12 or subsequentto forming the opening 16 in the dielectric layer 12, and then form thethick-film resistive mass 18 over the terminals 20. Alternatively, wells(not shown) could be formed in the surface of the substrate 10 prior toapplying the dielectric layer 12 or subsequent to forming the opening 16in the dielectric layer 12, with the terminals 20 being deposited in thewells prior to the resistive ink 14 being deposited in the opening 16.In this manner, the terminals 20 lie outside the plane of the resistivemass 18, though below the resistive mass 18 in contrast to that shown inFIG. 4.

From the above, those skilled in the art will appreciate that, becausethe photolithographically-formed opening 16 precisely determines thelateral dimensions (width and length) of the resistive mass 18, thephotolithographically-formed openings 24 precisely determine theelectrical length of the thick-film resistor formed by the resistivemass 18, and the thickness of the resistive mass 18 can be controlled bya thick-film deposition process, the resistor has little of thedimensional variability associated with conventional screen printedthick-film resistors of the prior art. Furthermore, the resistor can beas physically small as the edge definition attainable withphotodefinition techniques, enabling the precise formation of resistorshaving dimensions on the order of 100 micrometers.

While our invention has been described in terms of particularembodiments, it is apparent that other forms could be adopted by oneskilled in the art. For example, certain processing parameters could bemodified, appropriate materials could be substituted, or the process ofthis invention could be employed for different applications or otherprocesses. Accordingly, the scope of our invention is to be limited onlyby the following claims.

What is claimed is:
 1. A printed circuit board comprising: a substrate;a photoimageable layer on the substrate, the photoimageable layer havingan opening in a surface of the photoimageable layer, the opening beingcharacterized by a first opening dimension, a second opening dimensiontransverse to the first opening dimension, and a depth equal to athickness of the photoimageable layer, the photoimageable layer being apermanent dielectric layer of the printed circuit board; and a resistivemass within the opening, the resistive mass comprising carbon particlesin a polymeric matrix, the resistive mass having first resistordimension and a second resistor dimension that are approximately equalto the first opening dimension and the second opening dimension,respectively; the resistive mass having a resistive surface recessedbelow the surface of the photoimageable layer and a resistor thicknessless than the thickness of the photoimageable layer, the resistive masslying in a plane containing the photoimageable layer, the resistive masscontacting at least two electrical terminals disposed in a plane outsidethe plane containing the resistive mass and the photoimageable layer. 2.A printed circuit board as recited in claim 1, wherein the resistivemass contacts the substrate.
 3. A printed circuit board as recited inclaim 1, wherein the electrical terminals are disposed above theresistive mass and the photoimageable layer.
 4. A printed circuit boardas recited in claim 1, further comprising: a second photoimageable layeroverlying the photoimageable layer and the resistive mass; and twoopenings in the second photoimageable layer, the electrical terminalsdisposed in the two openings.
 5. A printed circuit board as recited inclaim 1, wherein the electrical terminals are disposed beneath theresistive mass.
 6. A printed circuit board as recited in claim 5,wherein the electrical terminals lie on the substrate and the resistivemass overlies and contacts the electrical terminals.